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Although only 167 examples were produced, the Kawanishi H8K was the most advanced flying-boat to achieve production status during World War II. Designed to meet a requirement issued in 1938 for a four-engine maritime reconnaissance flying-boat superior in all respects to the British Short Sunderland, the H8K1 prototype was first flown in January 1941, but initially possessed very poor water handling qualities. Extensive modifications were made and after successfully completing its service trials the aircraft was ordered into production as the Navy Type 2 Flying Boat Model 11, powered by four 1141kW Mitsubishsi Kasei 11 or 12 radials. Armament of these early aircraft was two 20mm cannon and four 7.7mm machine guns. With armour protection, selfsealing fuel tanks and a maximum speed of 433km/h, the new flying-boat was a considerable advance over the H6K.

It carried out its first operational mission in March 1942 when two aircraft of the Yokohama Kokutai set out from Wotje Atoll in the Marshalls to bomb Oahu Island (Pearl Harbor), putting down at French Frigate Shoals to refuel from a submarine. Arriving over the American base, the Japanese crews found heavy cloud and the raid was ineffective. As a longrange maritime reconnaissance aircraft, the H8K1 (codenamed 'Emily' by the Allies) with its 7,200km range, heavy armament and good performance proved a highly competent aircraft. 112 improved H8K2, with 1380kW Kasei 22 radials and armament increased to five 20mm cannon and four 7.7mm machine-guns, were built between 1943 and 1945. It was also equipped with ASV radar, being responsible for the sinking of at least three American submarines in the area north of the Philippines during the last 18 months of the war.

Additional to the maritime reconnaissance version, 36 H8K2-L boats were built in the last two years of the war, these being equipped as naval staff and troop transports capable of accommodating either 29 staff passengers or 64 fully armed troops. The progressively deteriorating war situation for Japan led to a run-down in production of flying boats during 1945 in favour of fighters for home defence, and later versions of the H8K were accordingly abandoned. Nevertheless this excellent aircraft saw considerable service, being flown by the 14th, 801st, 851st, 1001st, 1021st, Takuma, Toko, Yokohama and Yokosuka Chinjufu Kokutais.